During normal adult respiration, the amount of air inspired or expired per breath (tidal air) is approximately 500 ml.
After a normal tidal expiration, a further volume of air (the expiratory reserve-volume) can be expired--approximately 750-1000 ml in adults.
Vital capacity is the maximum volume of air which can be expelled from the lungs by a forceful effort, following a deliberate inhalation of the maximum volume possible. This vital capacity is normally of the order of 4.8 liters in men and 3.2 liters in women, but can be substantially reduced in asthmatics and people with other bronchial problems. The vital capacity and/or the ability to exhale thus provides a good indication of a subject's function.
Several devices exist which monitor a subject's ability to exhale, thus providing an indication of lung function. These devices employ a number of different methods which either directly or indirectly provide a measurement of expiratory volume; tidal reserve or more preferably vital capacity. For example, a subject may be asked to blow into a calibrated tube containing a spring-loaded device, such as a piston, which is moved along a calibrated scale according to the force provided by the exhaled air. The inertia of the spring-loaded device will affect the sensitivity of the equipment and may require that, for a valid measurement, a degree of force is needed which a severe asthmatic, or an otherwise respiratory-impaired individual, cannot provide.
An alternative method involves the use of a device containing a hot wire. Exhaled air passes over the hot wire, cooling it and a measure of the degree of cooling thus provides an indication of lung function. Again the device may not be sensitive enough for some objects. In addition the difficulty in accurately measuring the degree of cooling further affects the sensitivity.
A further method involves the use of rotary blades or a turbine. Exhaled air causes the blades to rotate and this motion is detected and translated into an indication of lung function. Turbines generally are heavy for their size and do not provide the required sensitivity. Rotary blade devices are generally arranged so that the exhaled air impinges on the edges of the blades (i.e. that is, perpendicular to their mountings) and, again, the inertia of the device will affect its sensitivity. This is particularly important for testing subjects with poor lung function.
Breath volumeters of this type are exemplified by the disclosure of German Patent Specification No. DE-A-1803325 (VEB MEDIZINTECHNIK LEIPZIZ) which describes a breath volumeter having a rotor fitted with a twin-armed vane arranged centrally below a semi-circularly shaped stator attached to the meter casing. The stator has a roughly semi-cross-section. In use, this arrangement suffers the disadvantage of developing a significant back-pressure in the device which is undesirable as it adversely affects the sensitivity and efficiency of the device.
The provision of back-pressure in such a device is essential if the parameter known as Force Expired Volume (FEV1) to be measured. This parameter, which is a measure of expired volume per second after initial exhilation and gives an indication of vital lung capacity and tidal reserve, is particularly important for assessing bronchitis and similar conditions. If back-pressure in a device is too low, a poor indication of vital capacity is received. Conversely, if back-pressure is too high, subjects having poor lung function will be unable to provide sufficient force to operate the device. Sensitivity of the device will also be adversely affected.
In all such breath volumeters, the inertia of the rotating blades or turbines is critical. High inertia results in slow acceleration of the blades and a continuation of rotation after the applied exhilation ceases will result. One problem associated with prior art devices is the exposure of the blades to moisture or sputum carried on the exhaled breath. A build-up of such deposits on the blade will naturally affect its inertia and consequently, the accuracy of the device. Devices where the breath is applied directly to the blades or where the air-flow is re-directed marginally to impinge the blades are particularly vulnerable. Where optical means are provided to measure device parameters, the deposits mentioned above also have a deleterious affect. Thus. Peak Flow readings, which are of particular interest for analysing asthma sufferers, may be affected.
U.S. Pat. No. 4,292,853, issued Oct. 6, 1981 to Williams et al discloses a respiratory air or gas flow volume indicating instrument having a slotted stator for directing exhaled air onto a rotor. By means of a barrier and a shaped gallery about the stator, air is constrained to flow in one direction about the stator, through the stator slots onto the blades or vanes of the rotor. The rotor spindle, to which the vanes are axially fixed, drives a pointer through a counting gear box to indicate the volume of air flow.
The U.S. Pat. No. 4,292,853 arrangement possesses the disadvantageous features referred to above. Firstly, the inertia of the rotor which is connected to a gearbox limits the allowable sensitivity and could not be used for accurate peak flow measurement. Secondly, air flow from the inlet can directly impinge the rotor allowing food particles, sputum or excessive vapour to adhere thereto. Additionally, the construction disclosed does not allow for easy disassembly and cleaning. Further, mechanical coupling of the rotor to a counting mechanism allows only for measurement of one parameter, in this case, volume flow. Finally, as the flow of air is constrained in the gallery, back pressure is relatively high (although stated to be tolerable) even though the flow path is relatively simple. The air flow path described is an inlet air flow perpendicular to the rotational axis of the rotor and an angular deflection into the gallery before being directed spirally inwards to impinge the rotor vanes.
U.S. Pat. No. 4,294,262, issued Oct. 13, 1981 to Williams et al describes a respirometer having a circular chamber having a rotor therein. The arrangement disclosed is essentially that forming part of U.S. Pat. No. 4,292,853 but having magnets mounted on radial arms rotatable with the rotor spindle. The magnets co-operate with at least one magnetic field sensitive device arranged to sense the rotation of the rotor. An electrical output is then available for manipulation to obtain the required respiratory parameters.
In addition to the disadvantages ascribed to the U.S. Pat. No. 4,292,853 arrangement, the affect of the magnets mounted on the radial arms is to increase the mass of the rotor, thus increasing inertia and decreasing sensitivity.
It will be appreciated that none of the prior art devices lend themselves to repeated cleaning or sterilisation.
It is an object of the present invention to overcome the disadvantages described above and to provide an apparatus for measuring respiratory performance which is more sensitive than prior art devices.
It is a further object of the invention to provide an improved an air-flow valve for use in a device for measuring respiratory function.
It is a yet further object of the invention to provide an air-flow measuring device which is easily cleaned or sterilised.